EUV condenser with non-imaging optics

ABSTRACT

An illumination system and condenser for use in photolithography in the extreme ultraviolet wavelength region has a first non-imaging optic element collecting electromagnetic radiation from a source and creates a desired radiance distribution. A second non-imaging optic element receives the electromagnetic radiation from the first non-imaging optic element and redirects and images the electromagnetic radiation. The electromagnetic radiation from the second non-imaging optic element is suitable for being received by other conventional optical surfaces to provide a desired radiance distribution with a desired angular distribution and desired shape. Facets are used to provide the desired illumination over the desired illumination field. Reflective facets may be placed on the second non-imaging optic, which can reduce the number of mirrors and increase efficiency. The condenser and illumination system are used in combination with a projection optic to project the image of a reticle or mask onto a photosensitive substrate, such as a semiconductor wafer. The condenser of the present invention provides an efficient condenser in a compact package and provides desirable illumination properties for imaging relatively small feature sizes of less than 0.13 microns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Application No. 09/236.888, filed onJan. 26, 1999, now U.S. Pat. No. 6,573,978 entitled EUV CONDENSER WITHNON-IMAGING OPTICS.

FIELD OF THE INVENTION

The preset invention relates generally to a condenser and illuminationsystems for projecting the image of a reticle onto a photosensitivesubstrate as used in photolithography in semiconductor manufacturing,and more particularly to a condense suitable for use in the extremeultraviolet or soft X-ray wavelengths having non-imaging optics forminga desired radiance and desired angular distribution.

BACKGROUND OF THE INVENTION

Photolithography is often used in the manufacture of many devices and inparticular, electronic or semiconductor devices. In a photolithographicprocess, the image of a reticle or mask is projected onto aphotosensitive substrate. As the element or feature size desired to beimaged on the photosensitive substrate becomes ever smaller, technicalproblems often arise. One of these problems is illuminating the reticleor mask so that its image can be projected onto the photosensitivesubstrate. As the element or feature size of semiconductor devicesbecome ever smaller, there is a need for photolithographic systemsproviding a resolution of less than 0.13 micrometers. In order toachieve the imaging of these relatively small element or feature sizes,shorter wavelengths of electromagnetic radiation must be used to projectthe image of a reticle or mask onto the photosensitive substrate.Accordingly, it is necessary for photolithographic Systems to operate atthe extreme ultraviolet wavelengths, below 157 nanometers, and into thesoft X-ray wavelengths, around 1 nanometers Additionally, projectionoptics having the required resolution and imaging capabilities oftenresult in utilization of a portion of a ring field. One such projectionoptic system used in photolithography is disclosed in U.S. Pat. No.5,815,310 entitled “High Numerical Aperture Ring Field Optical ReductionSystem” issuing to Williamson on Sep. 29, 1998, which is hereinincorporated by reference in its entirety. While the projection opticsystem disclosed therein can achieve a working resolution of 0.03microns, there are few illumination sources or illumination systems thatcan provide the required illumination properties for projecting theimage of the reticle or mask onto the photosensitive substrate. Anilluminating system is disclosed in U.S. Pat. No. 5,339,346 entitled“Device Fabrication Entailing Plasma-Derived X-Ray Delineation” issuingto White oil Aug. 16, 1994. Therein disclosed is a condenser for usewith a laser-pumped plasma source having a faceted collector lensincluding paired facets, symmetrically placed about an axis. Anotherillumination system is disclosed in U.S. Pat. No. 5,677,939 entitled“Illuminating Apparatus” issuing to Oshino on Oct. 14, 1997. Thereindisclosed is an illumination system for illuminating an object in anactuate pattern having a reflecting mirror with a parabolic-toxic bodyof rotation in the reflection type optical integrator having areflecting surface for effecting the critical illumination in themeridional direction and a reflecting surface for effecting the Kohlerillumination in the sagittal direction. Another illuminating system isdisclosed in U.S. Pat. No. 5,512,759 entitled “Condenser ForIlluminating A Ring Field Camera With Synchrotron Emission Light”issuing to Sweatt on Apr. 30, 1996, which is herein incorporated byreference in its entirety. Therein disclosed is a condenser comprisingconcave and convex spherical mirrors that collect the light beams, flatmirrors that converge and direct the light beams into a real entrancepupil of a camera, and a spherical mirror for imaging the real entrancepupil through the resistive mask and into the virtual entrance pupil ofthe camera. Another illumination system is disclosed in

U.S. Pat. No. 5,361,292 entitled “Condenser For Illuminating A RingField” issuing to Sweatt on Nov. 1, 1994. Therein disclosed is acondenser using a segmented aspheric mirror to collect radiation andproduce a set of actuate foci that are then translated and rotated byother mirrors so that all the arcuate regions are superposed at themask.

However, these prior illumination systems may not provide the desiredillumination and are relatively complicated. Additionally, many of thesesystems are relatively large, having many surfaces resulting in loss ofenergy. Some are also difficult: to align and may require adjustment.

Accordingly, there is a need for an improved illumination system andcondenser for use in the extreme ultraviolet that provides a desiredradiance over a predetermined field or area with a desired radiance andangular distribution for use in photolithography.

SUMMARY OF THE INVENTION

The present invention is directed to an illumination system comprising acondenser having non-imaging optic elements. A first non-imaging opticelement is used to collect light from a source and create a desired orpredetermined radiance distribution. A second non imaging optic elementreceives electromagnetic radiation from the first non-imaging opticelement and redirects the electromagnetic radiation into collimatednearly spherical or flat wavefronts. Facets placed at the pupil of theillumination system shapes the electromagnetic radiation and providesuniform illumination over a desired area. The facets may be provided onthe second non-imaging optic element. Additional objective optics may beutilized to further process the electromagnetic radiation or to relaythe electromagnetic radiation to the desired area at a reticle or mask,the image of which is projected onto a photosensitive substrate.

Accordingly, it is an object of the present invention to provide adesired radiance over a predetermined field or area.

It is a further object of the present invention to provide apredetermined angular and radiance distribution.

It is yet a further object of the present invention to increase theétendue of a source of electromagnetic radiation.

It is an advantage of the present invention that it is an efficientc-condenser for the desired wavelength.

It is a further advantage of the present invention that it is relativelycompact.

It is a feature of the present invention that non-imaging optic elementsare used.

It is another feature of the present invention that a relatively smallnumber of reflective surfaces are utilized.

It as yet a further feature of the present invention that a facetedoptic element is used.

These and other objects, advantages, and features will be readilyapparent in view of the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically illustrates the illumination system of the presentinvention.

FIG. 2 schematically illustrates optical element 18 illustrated in FIG.1.

FIG. 3 schematically illustrates the illumination system illustrated inFIG. 1 in combination with a projection optic.

FIG. 4A schematically illustrates an embodiment of the non-imaging opticelement used to form a desired radiance distribution.

FIG. 4B is a plan view graphically illustrating the desired radiancedistribution formed by the non-imaging optic element illustrated in FIG.4A.

FIG. 5A schematically illustrates an embodiment of the non-imaging opticelement in the shape of an axicon used to redirect the electromagneticradiation into collimated nearly spherical or flat wave fronts and anillumination field or area having a desired shape.

FIG. 5B is a plan view graphically illustrating the desired radiance andangular distribution having a desired arcuate illumination field or areaformed by the non-imaging optic element illustrated in FIG. 5A.

FIG. 5C schematically illustrates an embodiment of the non-imaging opticelement having a shape used to redirect the electromagnetic, radiationinto collimated nearly spherical or flat wave fronts and an illuminationfield or area having a desired shape.

FIG. 5D is an elevational view graphically illustrating a non-imagingoptical element having an odd asphere shaped base surface and Fresnelfacets.

FIG. 5E is a perspective view graphically illustrating the desiredradiance and angular distribution having a desired rectangularillumination field or area formed by a non-imaging optic element.

FIG. 6 graphically illustrates parameters utilized in calculatingreflective surfaces of the non-imaging optic elements.

FIG. 7A is a plan view illustrating a desired radiance and angulardistribution used in photolithography, and referred to as quadrupole.

FIG. 7B is a plan view illustrating a desired radiance and angulardistribution used in photolithography, and referred to as uniform.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates the illumination system 10 of thepresent invention utilizing non-imaging optics. An extreme ultrawide,radiation, EUV, source 12, which may be a laser plasma source, acapillary discharge tube or synchrotron, provides electromagneticradiation in the extreme ultraviolet to a non-imaging optic element 14.The collector or first non-imaging optic element 14 collects theelectromagnetic radiation from the source and creates a desired orpredetermined radiance distribution in the pupil of the illuminationsystem 10. The radiance distribution may be uniform, annular, quadrupleor other known or desired radiance distribution. Various radiancedistributions have known desirable properties for imaging differentpatterns onto a photosensitive substrate. Coatings having a gradedthickness profile may be place on the first non-Imaging optic element 14to improve reflectivity. The electromagnetic radiation reflected fromthe first non-imaging optic element 14 is collected by a shaper orsecond non-imaging optic element 18. The pupil of the illuminationsystem 10 is located near or adjacent the second non-imaging opticelement 18. Entering rays 16 are reflected from the second non-imagingoptic element 18 and exit as emerging rays 20. The non-imaging opticelement 18 redirects and collimates the electromagnetic radiation,illustrated as rays 20, into nearly spherical or flat wavefronts. Thisallows the use of conventional optical surfaces to be used in theremainder of the illumination system 10. The second non-imaging opticelement 18 may have a plurality of facets placed thereon providinguniform illumination over a desired area or illumination field. Thefacets are placed at the pupil, adjacent the second non-imaging opticelement 18, of the illumination system 10. Electromagnetic radiation,illustrated by rays 20, is reflected from the second non-imaging opticelement 18 and received by a conventional optical element 22.

Electromagnetic radiation is reflected from optical element 22,illustrated by rays 24, and received by a second conventional opticalelement 26, which in turn is reflected as rays 28 to illuminate areticle 30. The optical elements 22 and 26 may be conventional objectiveoptical elements. For example, optical element 22 may correct for comaand be relatively flat. Optical element 26 may be an oblate spheroid.The optical elements 22 and 26 are preferably anamorphic aspheres andare designed to image light reflected or scattered by the secondnon-imaging optical element 18 onto a reticle and into the entrancepupil of projection optics. The anamorphic departures are used tomaintain a constant round pupil across a reticle. Optical elements 22and 26 may relay or image illumination to the entrance pupil of theprojection optics.

This illumination fills the pupil with a predetermined radiance andangular distribution, which is referred to as pupil fill The presentinvention permits the pupil fill to be modified as desired to enhancethe imaging of a pattern on a reticle. Generally, desirable pupil fillsare well known, or can be determined relatively easily with a testreticle. Pupil fill or angular distribution may be more preciselyidentified by the radiometric term radiant intensity, which is commonlyexpressed in Watts per steradian. Lithographers often use pupil fill todescribe this quantity. However, confusion can arise because theradiance distribution at the pupil is sometimes also called the pupilfill. It is important to realize that the radiant intensity looking backfrom the reticle or mask from each field point will not necessarily bethe same as the radiance in the pupil. An example is if radiancedistribution were uniform, but the right half of the reticle or mask hada radiant intensity which was the right half of a circle and the lefthalf of the mask had a radiant intensity which was the left half of acircle.

Additionally, the present invention makes possible the illumination of adesired area on a reticle or mask, such as an arcuate, rectangle, orother shaped area. The present invention also provides a desiredradiance distribution, with a uniform radiance distribution usuallydesired, but may provide for a different profile or radiancedistribution if desired. The present invention also provides anilluminated region that is effectively self-luminous. Additionally, eachpoint in the illumination region is illuminated by the same desiredangular spectrum, whether it is uniform, annular, quadrupole, or otherdesired distribution. The present invention also provides that the exitpupil for each point in the illumination region is in the same locationto prevent telecentricity errors.

The first non-imaging optic element 14 collects electromagneticradiation from the source 12. The combination of the first and secondnon-imaging optic elements 14 and 18 forms the desired radiancedistribution at the illumination system 10 or condenser pupil and formsan image of the source 12. The second non-imaging optic element 18,functioning as a beam shaper. The beam shaper located at the pupil ofthe illumination system 10 or condenser scatters the incident radiationso that the image of the source fills the desired region of the reticleor mask 30. Each point of the reticle or mask 30 receives light orelectromagnetic radiation from a large number of discrete points in thepupil. The discrete points may number in the millions depending upon thesize of the facets 34 on the second non-imaging optic element 18. Whilethe pupil is not completely filled, because the source étendue is lessthan that accepted by the projection optics, the pupil fill is moreuniform than can be obtained using conventional segmented optics.Conventional optical elements limits the condenser to fill with onlytens of points or lines in the pupil. The improved pupil fill, of thepresent invention, is achieved by manufacturing the second non-imagingoptic element 18 with many individual facets using lithography and agray scale mask.

The illumination system or condenser is effectively a critical condenserwith a non-imaging optic element 18 or beam shaper at the stop todistribute the photons in a desired fashion across the reticle or mask30. In the preferred embodiment, the stop is at the second non-imagingoptic element 18, which serves simultaneously as a beam shaper and tore-image the source at infinity. Combining these two functions on thesecond non-imaging optic element 18 reduces the number of optics in theillumination system and improves system throughput.

FIG. 2 more clearly illustrates the second non-imaging optic element 18Preferably, facets 34 are formed on the base surface 32 of base 32 Itshould be appreciated that the facets 34 have been greatly exaggeratedin size and tilt for illustrative purposes. The facets 34 are shaped andtilted such that the angular spectrum reflected from the non-imagingoptic element 18 forms the predetermined or desired shape orillumination field at the reticle or mask when imaged by the opticalelements 22 and 26, illustrated in FIG. 1. Facets 34 are chosen so thatthe angular spectrum produces an arcuate region when imaged. Diffractionfrom the facets will increase the étendue. Facets 34 should be smallenough to provide reticle plane uniformity, and large enough to minimizeedge scattering. For example, a one hundred and twenty millimeterdiameter or dimension non-imaging optical element may have tenmicrometer dimension facets. This provides good uniform illumination andpupil fill. The facets may be reduced in size to at least fourmicrometers. The facets 34 may preferably form a Fresnel surface.

The beam shaper or second non-imaging optical element 18 receiveselectromagnetic radiation or light from the collector or firstnon-imaging optical element 14 and redirects it over the desiredillumination field. Conceptually, this can be divided into two separatetasks: reimaging of the source 12 which is accomplished by the basesurface 32 and blurring the source image over the illumination fieldwhich is accomplished by engineering a scattering surface composed ofmany small facets 34.

The combination of the collector or first non-imaging optical element 14and the beam shaper or second non-imaging optical element 18 surfacesreimages the source, at finite or infinite conjugates. The shaper basesurface 32 is determined by 1) solving the differential equations thatdefine the collector or first non-imaging optical element 14, 2)calculating the angle of incidence of the rays θ at the beam shapersurface, and 3) calculating the slope of the shaper surface necessary toredirect the incident radiation towards a finite or an infiniteconjugate. The slopes of the shaper surface may be integrated to givethe surface profile or the slopes may be directly integrated into theray tracing software. The base beam shaper surface can be polishedconventionally and the scattering function applied on the base surface.However, the preferred implementation is to manufacture a Fresnelsurface using the same small, preferably four to ten micron squarefacets, that are required for the scattering.

For the Fresnel surface, each base facet tilt redirects the chief rayfor that facet (i.e. one that starts at the center of the source to andgoes to the center of said facet) to the image of the source. TheFresnel facet tilt is added to that scattering tilt, as described below.Using a Fresnel base surface simplifies the manufacture of thescattering pixels, because the substrate is not curved, standardlithographic processing can be used.

In order to illuminate the desired region at the mask, the pupil (i.e.the beam shaper surface) is divided into many small facets. Each facetdirects light to a different portion of the illumination field. Thisengineered scattering surface allows the illumination of arbitrarilyshaped regions. With sufficiently small facets, each point in theillumination field can receive light from thousands or millions ofpoints distributed randomly over the pupil. The random distribution inthe pupil ensures the angular distribution of the radiation or radiantintensity at each point on the reticle is substantially the same. Thedeviation of each facet from the base surface is determined by focallength of the remaining conventional optical surfaces and the paraxialimaging equation.

Referring to FIGS. 1 and 2, the illumination system 10 of the presentinvention illuminates a reticle with an image of a predeterminedillumination field or area having desired radiance and a desired angulardistribution. This illumination is desirable for photolithographicapplications. Additionally, the illumination system 10 of the presentinvention increases the étendue of the electromagnetic radiation sourceby evenly distributing the available power across the illumination fieldor arcuate area. The étendue of an optical system is a geometricalquantity related to the cross sectional area of the source and theangular subtents collected by the aperture. Laser plasma and capillarydischarge sources are generally small, less than approximately onemillimeter. Synchrotrons have small angular extends, which must be takeninto account in the design process. As a result, it is often necessaryto increase the étendue of the source. The present invention achieves adesired illumination pattern, a desired angular distribution, and highefficiency, all in a compact package. The present invention ispreferably applicable to irradiate a portion of a ring field or arcuateregion of a projection optic with extreme ultraviolet electromagneticradiation. One such projection optic system is disclosed in U.S. Pat.No. 5,815,310 entitled “High Numerical Aperture Ring Field OpticalReduction System” issuing to Williamson on Sep. 29, 1998, which isherein incorporated by reference in its entirety.

Referring to FIGS. 1 and 2 illustrating the illumination system 10 ofthe present invention, a relatively small source 12, such as a laserplasma source or a capillary discharge tube emits electromagneticradiation with the desired wavelength in the extreme ultraviolet region.The collector or first non-imaging optic element collects the energy orelectromagnetic radiation and forms a desired radiance distributionprovided to the shaper or second non-imaging optic 18. The non-imagingsurfaces of the non-imaging optic elements are the solution todifferential equations. These differential equations are well known tothose skilled in the art and are disclosed in a book entitled “ComputerAided Optical Design of Illumination an Irradiating Devices” by Kusch,and published by Asian Publishing House, Moscow, 1993. Using equationsfound in this book non-imaging optic elements are readily designed forpoint sources with rotationally symmetric angular intensity androtationally symmetric pupil fills. For a finite sized source andnon-rotationally symmetric systems more general equations may be used.Such equations are well known to those skilled in the are and may befound in an article entitled “Formulation of a Reflector Design Problemfor a Lighting Fixture” by J. S. Schruben published in the Journal ofthe Optical Society of America, Vol. 62, No. 12, December 1972 and in anarticle entitled “Tailored Reflectors for Illumination” by D. Jenkinsand R. Winston published in Applied Optics, Vol. 35, No. 10, April 1996.The reflectivity of the coatings and the angular intensity distributionof the source are both taken into account when designing the non-imagingoptic surfaces. The base surface of second non-imaging optic element 18is non-imaging and collimates electromagnetic radiation from the firstnon-imaging optic element 14 so as to be reflected by the secondnon-imaging optic element 18 in a parallel bundle. On the surface 32 ofthe second non-imaging optic element 18 is formed an array of facetsshaped and tilted such that the angular spectrum reflected from thesecond non imaging optic element 18 forms the desired shape at thereticle or mask when imaged by the remaining conventional optics 22 and26 of the present invention. The second non-imaging optical element 18may be fabricated using lithographic techniques.

FIG. 3 illustrates the extreme ultraviolet illumination system 10, asillustrated in FIGS. 1 and 2, in combination with an extreme ultravioletprojection optic 36, such as that disclosed in U.S. Pat. No. 5,815,310.Illumination system 10 provides a desired radiance and a desired angulardistribution in a predetermined illumination field, such as a portion ofa ring field or arc, for illuminating the reticle 30. Reticle 30 is areflective reticle. As a result, the electromagnetic radiation strikesthe reticle 30 slightly off axis with respect to a line normal to thereticle 30. The electromagnetic radiation is collected and reflected bya first mirror 38, which is collected and reflected by a second mirror40, which is collected and reflected by a third mirror 42, which iscollected and reflected by a fourth mirror 44, which is collected andreflected by a fifth mirror 46, which is collected and reflected by asixth mirror 48, which projects the image of the reticle onto aphotosensitive substrate 50. Sufficient clearance is provided betweenthe EUV illumination system 10 and the projection optic 36 to permit thereticle 30 and the photosensitive substrate 50 to be scanned in aparallel direction such that- the illumination field projects the imageof the reticle over a predetermined area of the photosensitivesubstrate. The reticle 30 and the photosensitive substrate 50 arepositioned for parallel scanning. All of the optical elements arepositioned to not interfere with any needed stages for the movement ofthe reticle 30 and photosensitive substrate 50. Accordingly, the entiresystem, illumination system 10 and projection optic 50, are preferablypositioned entirely between the reticle 30 and the substrate 50.

FIG. 4A schematically illustrates the profile of a collector or firstnon-imaging optical element 214. The collector or first non-imagingoptical element 214 may be axially symmetrical. The collector ornon-imaging optic element 214 has a curved reflective surface 215. Thecurved reflective surface 215 is calculated or determined based upon thedesired radiance distribution at the pupil. A relatively small EUVsource 212 is positioned near the origin of the non-imaging opticalelement 214. The electromagnetic radiation from the source 212 iscollected and reflected by the first non-imaging optical element 214.The reflected electromagnetic radiation forms a desired radiancedistribution.

FIG. 4B graphically illustrates one such radiance distribution. Anannular radiance distribution 52 is illustrated. The annular radiancedistribution 52 is characterized by a surrounding dark field 54 with acontained annular illumination 56 and dark center 58. While an annularradiance distribution 52 has been illustrated, any desired radiancedistribution may be obtained depending upon the calculated profile orshape of surface 215 of the non-imaging optical element 214. Forexample, top hat, quadrupole, uniform or other known desired radiancedistribution.

FIG. 5A illustrates a shaper or second non-imaging optical element 218.The second non-imaging optical element 218 is another embodiment of thesecond non-imaging optical element 18 illustrated in FIGS. 1 and 2. Thesecond non-imaging optic element, 218 is formed on a substrate or base231. The base 231 has a surface 232. The surface 232 may have a shape ofa Fresnel axicon. While FIG. 5A illustrates the second non-imaging opticelement 218 as a conical section, the conical surface of an axicon maybe placed on a conventional round lens for mounting. While FIG. 5Aillustrates an axicon, it should be appreciated that the secondnon-imaging optical element 218 may have other shapes depending upon thesource and desired radiance or angular distribution. For example, ifangular distribution of the source arid the pupil fill is rotationallysymmetric, the second non-imaging optic element has a shape of anodd-asphere. When there is no rotational symmetry, the secondnon-imaging optic element will have an unusual shape. The unusual shapeis determined to provide the function of collimating the rays. Thesurface 232 has a plurality of reflective angled or tilted facets 234thereon. The surface 232 in combination with the plurality of facets 234shape or redirect the electromagnetic radiation collimating it andforming spherical or flat wavefronts having a uniform illumination overa desired illumination area or field. The second non-imaging opticalelement 218 may have a relatively large number of reflective facets 234,with each facet 234 having a dimension of approximately ten to fourmicrometers. While ten to four micrometers is the most useful range offacet sizes, smaller and larger facets would also work. The angle ortilt of each facet 234 is calculated to provide an arcuate illuminationarea or field with uniform illumination and the desired radiancedistribution. The facets 234 may be made by etching or otherphotolithographic techniques. The uniform illumination has a desiredradiance distribution created by the collector or first non-imagingoptical element 14 or 214 illustrated in FIGS. 1, 3, and 4A.

FIG. 5B graphically illustrates a desired illumination area or fieldhaving an arcuate shape. Additionally, graphically illustrated is anannular angular distribution 152. The annular angular distribution 152has a dark field 154 surrounding an annular illumination 156 and darkcenter 158. While the annular angular distribution has been graphicallyillustrated, it will be understood by those skilled in the art that theangular distribution appears annular from any point in the illuminationarea or field. Therefore, the illumination area or field is uniformlyilluminated with annular radiance and angular distribution.

FIG. 5C is a perspective view illustrating another embodiment of thesecond non-imaging optical element. In this embodiment the non-imagingoptical element 318 has a base 331 with a base surface 332 formedthereon. The base surface 332 may have an unusual shape. The unusualshape is determined to provide the function of collimating the rays.Reflective facets 334 are formed on the base surface 332. The positionand tilt of the facets 334 are determined so as to provide a desiredillumination over a desired illumination field.

FIG. 5D is a side elevational view illustrating another embodiment ofthe second non-imaging optic element. In this embodiment the secondnon-imaging optic element 418 has a base 431 with a base surface 432having the shape of an odd asphere. An odd asphere is a surface whosesag can be written as a function of the radial distance on the vertexplane, the locus of points (x, y, z(r)), where r=(x²+y²)^(1/2). Anaxicon is a special case of an odd asphere. An odd asphere isnon-imaging because it does not image points to points at anyconjugates. For this reason, the first non-imaging optic element orcollector may also be an odd asphere. On base surface 432 are placedfacets 434. The facets are positioned so as to provide a desiredillumination over a desired area or illumination field.

FIG. 5E is a perspective view schematically illustrating a rectangularillumination field 360. The illumination field or area 360 has a desiredradiance distribution and desired angular distribution or radiantintensity. Radiance distribution is commonly expressed in Watts permeter squared. It is important to realize that the radiant intensitylooking back from the reticle or illumination field to the pupil fromeach field point, will not necessarily be the same as the radiancedistribution in the pupil. The illumination field 360 is graphicallyillustrated with a uniform radiance distribution 352. Severalillumination cones 357 are illustrated, but it should be appreciatedthat the illumination cones 357 as well as the uniform radiancedistribution 352 are across the entire illumination field 360.Accordingly, a desired radiance distribution may be obtained with adesired pupil fill or radiant intensity in any desired shape ofillumination field.

FIG. 6 graphically illustrates the parameters utilized to determine ordefine the surface of a non-imaging optic element. Point 512 representsa source, line 514 represents a reflector surface, and line 519represents the illumination plane. The non-imaging optic element may bedefined by differential equations which relate the angular distributionof the point source to an arbitrary output radiance distribution. Bynon-imaging it is meant an optical element that does not have a finitenumber of foci. A system of differential equations define the surface.The solution suitable for determining the surface of the non-imagingoptic element, such as that illustrated in FIG. 1 as elements 14 and 18or 214, 218, 318, and 418 in FIGS. 4A, 5A, 5C, and 5D may be obtained byreferring to the following differential equations: $\begin{matrix}{\frac{\phi}{x} = {\pm \frac{{xE}(x)}{{\rho (\alpha)}{I(\phi)}{\sin (\phi)}}}} \\{\frac{r}{\phi} = {\frac{\phi}{x}{{r\tan}(\alpha)}}}\end{matrix}$

where;

E(x) is radiance;

ρ is the reflectance;

I(φ) is the intensity;

r is a radial distance between a source and the non-imaging opticalelement reflective surface;

φ is an angle between the radial distance r and a perpendicular linefrom an illumination plane extending through the source,

θ is an angle between a perpendicular line from an illumination planeand a ray reflected from the non-imaging optical element reflectivesurface;

x is the distance along an illumination plane from the intersection ofthe illumination plane and a perpendicular line extending through thesource and a ray reflected from the non-imaging optical elementreflective surface, and

α=(ρ−θ)/2

Additionally, referring to FIG. 6,

χ=r sin(φ)+ tan(θ)[r cos(φ)+D]

Accordingly, many desired rotationally symmetric radiance distributionsmay be achieved by the fabrication of a reflective surface defined bythe above differential equations. The reflective surface determined toproduce the desired radiance distribution forms the first non-imagingoptic element. Accordingly, the image of the source is not formed. Bynon-imaging it is meant that the image of the source is not formed bythe non-imaging optic element. Therefore, by solving the abovedifferential equations, a reflector or collector can be described by aninterpolating function, assuming a point source, which can accommodateany irradiation distribution as long as x(ρ) increases with ρ. As can beappreciated from the above equations, the reflectance of the EUVcoatings can be utilized to create the desired radiance distribution.The second non-imaging optic, element 18 in FIGS. 1 and 3 and elements218, 318, and 418 in FIGS. 5A, 5C, and 5D, is used to redirect theincident radiation such that it fills the illumination area or fieldwhen imaged by the remaining conventional optical elements.

FIGS. 7A-B illustrate various radiance distributions desirable for usein photolithography depending upon the dominant characteristics of theintended pattern being reproduced. FIG. 7A graphically illustrates aquadrupole radiance distribution 254. The quadrupole radiancedistribution has a dark field 254 and four illumination fields 256. FIG.7B graphically illustrates a uniform radiance distribution 352.

Accordingly, the present invention, by using a first non-imaging opticelement to provide the desired radiance distribution in combination witha second non-imaging optic element, to provide nearly spherical or flatwavefronts or collimated electromagnetic radiation permits the use ofconventional optical surfaces in the remainder of the illuminationsystem. This makes possible a desired radiance having desired angulardistribution. Additionally, the use of facets placed at the pupil orstop and preferably on the second non-imaging optic element providesuniform illumination over the desired illumination field. While thefacets may be transmissive, they are preferably reflective and placed onthe surface of the second non-imaging optic element. Refractive ortransmissive elements may unduly attenuate or absorb the EUVelectromagnetic radiation. The present invention also permits a changein numerical aperture by masking the pupil or changing the first andsecond non-imaging optical elements. This provides a desired sigma orcoherence, ratio of numerical apertures, for the system.

Additionally, although the preferred embodiment has been illustrated anddescribed, it will be obvious to those skilled in the art that, variousmodifications may be made without departing from the spirit and scope ofthis invention.

What is claimed is:
 1. An extreme ultraviolet condenser comprising: anon-imaging mirror; a non-planar optical element that receiveselectromagnetic radiation reflected from said non-imaging mirror; and afaceted optical element near a stop of the condenser.
 2. The extremeultraviolet condenser of claim 1, wherein said faceted optical elementis formed on said non-planar optical element.
 3. An extreme ultravioletcondenser as in claim 1, wherein said non-imaging mirror has variedreflectance.
 4. The extreme ultraviolet condenser of claim 1, furthercomprising at least two mirrors that receive the electromagneticradiation from said non-planar optical element.
 5. The extremeultraviolet condenser of claim 4, wherein said at least two mirrorsimage said non-planar optical element onto a reflective reticle.
 6. Theextreme ultraviolet condenser of claim 4, wherein said at least twomirrors comprise anamorphic aspheres.
 7. The extreme ultravioletcondenser of claim 1, wherein the condenser generates an arcuateillumination field.
 8. A condenser for use with extreme ultravioletwavelengths comprising: a reflective collector that does not image asource; a non-planar shaper that receives electromagnetic radiation fromsaid collector and collimates the electromagnetic radiation into anillumination field; a plurality of facets on said non-planar shaper; andan objective that forms an image of the illumination field on a reticle.9. The condenser of claim 8, wherein said non-planar shaper is anon-imaging optical element.
 10. The condenser of claim 8, wherein saidnon-planar shaper has an odd aspheric surface.
 11. The condenser ofclaim 8, wherein said non-planar shaper is a fresnel axicon.
 12. Thecondenser of claim 8, wherein each of said plurality of facets arereflective.
 13. The condenser of claim 12, wherein each of saidplurality of facets has a surface dimension of at least fourmicrometers.
 14. The condenser of claim 8, wherein said objectivecomprises anamorphic aspheres.
 15. The condenser of claim 8, wherein theillumination field is an arcuate illumination field.
 16. A condenser foruse in an extreme ultraviolet illumination system comprising:non-imaging means for collecting electromagnetic radiation from asource; non-planar shaper means that receive electromagnetic radiationfrom said non-imaging means, collimate the electromagnetic radiation andform an illumination field; and facet means, associated with saidnon-planar shaper means, that form a uniform illumination over theillumination field.
 17. The condenser of claim 16, wherein theillumination field is an arcuate illumination field.
 18. A condenser foruse in an extreme ultraviolet illumination system comprising:non-imaging means for collecting electromagnetic radiation from a sourceand forming a desired radiance distribution; shaper means, positioned toreceive electromagnetic radiation from said non-imaging means, forcollimating the electromagnetic radiation and forming an illuminationfield having a desired shape; and facet means, associated with saidshaper means, for forming a uniform illumination over the illuminationfield.
 19. The condenser of claim 18, wherein said non-planar shapermeans has an odd asphere shaped surface.
 20. The condenser for of claim18, wherein said non-planar shaper means comprises an axicon.
 21. Thecondenser of claim 18, wherein said facet means comprises a plurality ofreflective facets on said non-planar shaper means.
 22. The condenser ofclaim 18, wherein the illumination field is an arcuate illuminationfield.
 23. A condenser for use in the extreme ultraviolet wavelengthregion comprising: a collector having a reflective surface defined by:$\begin{matrix}{\frac{\phi}{x} = {\pm \frac{{xE}(x)}{{\rho (\alpha)}{I(\phi)}{\sin (\phi)}}}} \\{\frac{r}{\phi} = {\frac{\phi}{x}{{r\tan}(\alpha)}}}\end{matrix}$

where E(x) is radiance, ρ is reflectance, r is a radial distance betweena source and a reflective surface of a non-imaging mirror, φ is an anglebetween the radial distance r and a perpendicular line from anillumination plane extending through the source, θ is an angle between aperpendicular line from an illumination plane and a ray reflected fromthe reflective surface of the non-imaging mirror, α is (φ−θ)/2, and x isthe distance along an illumination plane from an intersection of theillumination plane and a perpendicular line extending through the sourceand a ray reflected from the non-imaging mirror; a non-planar shaperthat receives electromagnetic radiation reflected from said collector,and collimates the electromagnetic radiation into an illumination field;a plurality of facets on said non-planar shaper; and an objective thatforms an image of the illumination field on a reticle.
 24. The condenserof claim 23, wherein said non-planar shaper is an axicon having aconical surface, and said plurality of facets are formed on the conicalsurface.
 25. The condenser of claim 23, wherein the illumination fieldis an arcuate illumination field.
 26. A condenser for use in an extremeultraviolet wavelength region comprising: a collector having areflective surface defined by: $\begin{matrix}{\frac{\phi}{x} = {\pm \frac{{xE}(x)}{{\rho (\alpha)}{I(\phi)}{\sin (\phi)}}}} \\{\frac{r}{\phi} = {\frac{\phi}{x}{{r\tan}(\alpha)}}}\end{matrix}$

where E(x) is radiance, ρ is reflectance, I(φ) is the intensity, r is aradial distance between a source and a non-imaging mirror, φ is an anglebetween a radial distance r and a perpendicular line from anillumination plane extending through the source, θ is an angle between aperpendicular line from an illumination plane and a ray reflected fromthe non-imaging mirror, α is (φ−θ)/2, and x is the distance along anillumination plane from an intersection of the illumination plane and aperpendicular line extending through the source and a ray reflected fromthe non-imaging mirror; a shaper that receives electromagnetic radiationreflected from said collector, and shapes and collimates theelectromagnetic radiation into an illumination field; a plurality offacets formed on said shaper, said plurality of facets providing adesired radiance within the illumination field; and an objective thatforms an image of the illumination field on a reticle.
 27. The condenserof claim 26, wherein said shaper is an axicon having a conical surfaceand said plurality of facets are formed on the conical surface.
 28. Thecondenser of claim 26, wherein the illumination field is an arcuateillumination field.
 29. An illumination system for use inphotolithography comprising: a source of electromagnetic radiation witha wavelength less than two hundred nanometers; a non-imaging reflectivecollector that receives the electromagnetic radiation from said source;a non-planar shaper that receives the electromagnetic radiation fromsaid non-imaging collector, collimates the electromagnetic radiation andforms an illumination field; and an objective that receives theelectromagnetic radiation from said non-planar shaper, and images theillumination field such that an image of a reticle is projected onto aphotosensitive substrate.
 30. The illumination system of claim 29,wherein said non-planar shaper comprises an axicon having a conicalsurface, and a plurality of reflective facets formed on said conicalsurface.
 31. The condenser of claim 29, wherein the illumination fieldis an arcuate illumination field.
 32. A method of illuminating a reticlewith extreme ultraviolet electromagnetic radiation comprising: forming aradiance distribution of electromagnetic radiation with a non-imagingmirror; collecting said electromagnetic radiation reflected from saidnon-imaging mirror; collimating said electromagnetic radiation collectedfrom said non-imaging mirror; and shaping the collimated electromagneticradiation with a non-planar shaper such that an imaged illuminationhaving a desired angular distribution is formed over an illuminationfield.
 33. The method of claim 32, wherein the illumination field is anarcuate illumination field.
 34. A condenser for use with extremeultraviolet electromagnetic radiation comprising: a non-imaging mirroradjacent to a source of the electromagnetic radiation; a non-planarnon-imaging optical element having a base surface that receives theelectromagnetic radiation reflected from said non-imaging mirror andcollimates the reflected electromagnetic radiation; a plurality ofreflective facets adjacent to said base surface; and imaging optics thatreceive the electromagnetic radiation reflected from said plurality ofreflective facets and project an image of a reticle onto aphotosensitive substrate.
 35. The condenser of claim 34, wherein the anarcuate illumination field.
 36. A condenser for use with extremeultraviolet electromagnetic radiation comprising: a non-imaging mirroradjacent to a source of electromagnetic radiation; an optical elementhaving a base surface that receives the electromagnetic radiationreflected from said first non-imaging mirror and collimates thereflected electromagnetic radiation; a plurality of reflective facets onsaid base surface; and imaging optics that receive the electromagneticradiation reflected from said plurality of reflective facets and imagethe electromagnetic radiation, whereby desired illumination field andpupil fill are used to project an image of a reticle onto aphotosensitive substrate.
 37. The condenser of claim 36, wherein thereflected electromagnetic radiation includes an arcuate illuminationfield.
 38. An extreme ultraviolet condenser comprising: a mirror withouta finite number of foci; a non-planar optical element that receives theelectromagnetic radiation reflected from said mirror, said non-planaroptical element having a base surface that collimates theelectromagnetic radiation; and a faceted optical element near a stop ofthe condenser.
 39. The condenser of claim 38, wherein the desiredillumination field is an arcuate illumination field.
 40. An illuminationsystem for use in extreme ultraviolet wavelength region comprising: asource of electromagnetic radiation having a wavelength less than twohundred nanometers; a reflective collector without a finite number offoci; a non-planar shaper that receives the electromagnetic radiationreflected from said reflective collector, collimates the electromagneticradiation and forms an illumination field; and an objective thatreceives the electromagnetic radiation from said non-planar shaper andimages the illumination field so that an image of a reticle is projectedonto a photosensitive substrate.
 41. The system of claim 40, wherein theillumination field is an arcuate illumination field.